Phalaenopsis dying

[isotestardo]

Hello everyone, first time here for me.
I've been growing Phalaenopsis for years , but in the last year I lost two of them in the same way. Now , it's starting again with another , leaves turn yellow as the main stem becomes brown and dry from the bottom to the top . In the end the leaves drop down and the plant dies.
I don't know what the problem is, it never happened the years before.
I water them from the top about once a week ( it depends on the season ) , I fertilize once a month ( 20-20-20 ) .
Does anyone have an idea ? I'd like to save this one, but more importantly I wouldn't lose more of them.

Thank you